1. Field of the Invention
The present invention relates to a miniature electric motor in which the area of a stator is defined in relation to a coil.
2. Description of the Related Art
FIG. 13 shows a cylindrical stepping motor that is currently available as a miniature motor. Stator coils 105 are coaxially wound around respective bobbins 101, and the bobbins 101 are clamped in the direction of the axis of the motor by two stator yokes 106. The stator yokes 106 have respectively stator teeth 106a and stator teeth 106b arranged alternately circularly around the inner circumferences of the bobbins 101. The stator yokes 106 integrally formed with the stator teeth 106a and 106b are fixed to a pair of cases 103, constructing a stator 102.
One of the pair of cases 103 supports a flange 115 and one bearing 108, and the other of the pair of cases 103 supports the other bearing 108. A rotor 109 includes a rotor magnet 111 fixed on a rotor shaft 110, and there is a radial air gap between the rotor magnet 111 and the stator yoke 106a of the stator 102. The rotor shaft 110 is rotatably supported by two bearings 108. The miniature stepping motor thus constructed for driving a camera lens is disclosed in Japanese Patent Laid-Open No. 3-180823.
Since the above conventional miniature stepping motor includes the cases 103, bobbins 101, stator coils 105, stator yokes 106 and the like coaxially arranged surrounding the rotor, the size of the motor becomes large. The magnetic flux generated with the stator coils 105 conducting currents chiefly passes the end face 106a1 of the stator tooth 106a and the end face 106b1 of the stator tooth 106b, as shown in FIG. 14. The magnetic flux are thus unable to efficiently act on the rotor magnet 111, failing to result in a high motor output power.
U.S. patent application Ser. No. 08/831,863 which is assigned to the assignee of the present invention discloses a technique that resolves this problem.
The motor proposed in the above applications includes a cylindrical rotor composed of a permanent magnet that is magnetized in alternate polarities equally and circularly spaced around, a first coil, a rotor and a second coil in the direction of the rotor. A first external magnetic pole and a first internal magnetic pole, both excited by the first coil, are arranged to face the outer circumference and the inner circumference of the rotor, respectively, and a second external magnetic pole and a second internal magnetic pole, both excited by the second coil, are arranged to face respectively the outer circumference and the inner circumference of the rotor. A rotating shaft is projected out of the cylindrical permanent magnet.
Such a motor has reduced external dimensions and provides high output power. However, the small radial dimension of the internal magnetic poles presents difficulty machining toothed poles. Furthermore, a need exists for a motor of a small radial dimension that provides a reliable output with a small runout.
U.S. patent application Ser. No. 08/994,994, which is assigned to the assignee of the present invention discloses a motor having an internal magnetic pole which is easy to machine. Furthermore, U.S. patent application Ser. No. 09/22474 which is assigned to the assignee of the present invention discloses a motor having a rotor which provides a reliable output with a small runout and in which transmission means, such as a gear and a pulley, is easily mounted to a rotating shaft of a small radial dimension.
There is a need for a miniature and high-performance motor that takes advantage of a relationship between a coil and a stator.